Optically transparent polymeric substrates, such as polymethylmethacrylate (PMMA) and polycarbonate (PC), are of interest as replacement for glass in applications where there is potential for impact, such as vehicle windows or safety goggles. Of these, polycarbonate (PC) is a choice of material for use as windows in building structures and automobiles. Unfortunately, water forms droplets on a surface of polycarbonate due to its high water contact angle of about 95°. When a surface of polycarbonate gets wet such as in the event of rain, for example, this may result in streaking of the polycarbonate surface. Upon drying, the polycarbonate surface becomes dirty due to segregation of dirt along the edges of the streaks or droplets as a result of the coffee-ring effect.
To resolve this, surface modification to obtain superhydrophilic surfaces having water droplet contact angle smaller than 10° have attracted great attention. A superhydrophilic coating may be applied to a surface of the polycarbonate to allow water to spread quickly, thereby avoiding streaking of dirt on surfaces to allow formation of cleaner surfaces. Unfortunately, as the surface becomes contaminated with organics from the surroundings such as sweat or fumes with time, efficacy of this mechanism decreases and the surface would need to be cleaned chemically or physically.
Superhydrophilic surfaces may be obtained using photocatalytically active materials such as titanium dioxide (TiO2), which become superhydrophilic upon illumination of UV light. These superhydrophilic films induced by photocatalytic activity, however, lose their superhydrophilicity a few minutes to hours after removal of the UV irradiation, or after storing in the dark. Furthermore, in case of titanium dioxide, the crystalline titanium dioxide coats may only be applied on surfaces at temperatures above 600° C. (e.g. Pilkington Activ). This limits application of the titanium dioxide coats to only materials that are able to withstand the high temperatures, hence temperature sensitive substrates, for example, polycarbonate which has a glass transition temperature of about 150° C., and organic substrates cannot be used. State of the art superhydrophilic films are also not photocatalytically active.
In view of the above, there remains a need for improved films or coatings and method of preparing the films or coatings that overcome or at least alleviate one or more of the above-mentioned problems.